Pro-combi isn't a make, its a brand name for a well known distributor (a supplier of independent merchants), and will be a badged version of another boiler. I think that Ideal might be making them at the moment, but in the past it has been Vokera and possibly others.
There are some good warranty deals around at present - I appreciate that you are on a budget, but the Ideal Independent + (note the plus sign at the end) is reasonably priced and carries 5 a five year warranty. It is a slightly re-engineered version of the more expensive Ideal Logic +. If you leave the "+" off either, then you save a bit of money and get the same boiler with just two year warranty.
Similarly Main boilers are generally a lower-cost option than their parent company's main brands, Baxi or Potterton. 2 years is probably the maximum warranty you will get on a Main.
The market leader (biggest selling) brands are Worcester Bosch and Vaillant. Both have entry level models (Worcester Junior 24i and Vaillant Ecotec Pro 24) but in price terms, they are mid-market. Its a bit like buying an "entry level" BMW or Mercedes - cheap compared to the rest of that range, but still dearer than the equivalent Hyundai or Kia.
As for 24kw or 30kw, as others have said, it will make no difference to your heating (either is more than capable of heating 8 small rooms), but you will notice the difference on the flow-rate of the hot tap, particularly on the bath. Flowrate at a given temperature rise is directly proportionate to the power of the boiler - so the extra 6kw difference between a 30kw and a 24kw model means that the higher output will take proportionately less time to fill a bath.
Tip - if you do go for a long warranty option, be sure to have it serviced annually. If you save a few quid by not bothering with the service, it will cost you 4 years down the line when a part fails and the warranty is invalid because you havent had it serviced according to the manfacturers instructions.
Final tip - every installer has brands they like and brands they hate - usually driven by a pattern of recent experiences, good or bad. On the upside, if you choose an installer that you trust and go with his recommendation, he is likely to understand that product well, may have been on the manufacturers training course, and is therefore more likely to be able to diagnose and fix any niggles. He may also have a relationship with the rep or the local service engineer - all these things tend to work in your favour if there is a problem.
